1. Field of the Invention
The present invention relates to a method and a machine for producing an optical compensation sheet used for STN liquid crystal display, TFT liquid crystal display or OCB liquid crystal display.
2. Description Related to the Prior Art
A liquid crystal display is widely used, and the demand, of a polarizing filter and an optical compensation sheet used in the polarizing filter becomes higher. In a producing process of the optical compensation sheet, a polymer solution is applied onto a transparent polymer film sheet base, and dried thereafter to form a polymer layer. Thus the polymer film sheet base and the polymer layer construct a film sheet. Thereafter a rubbing action is performed to the film sheet to form the polymer layer into an alignment layer. Then, a solution containing liquid crystal is applied onto the alignment layer and dried to obtain the optical compensation sheet. All of these steps in the producing process are often made continuously. Otherwise, in accordance with maintenance of the product line and conditions, for example, for exchanging materials at each step, the film sheet is rolled to a sheet roll between forming and rubbing the polymer layer.
Recently, the many sorts of the optical compensation sheet are required. For example, it is necessary to optionally set the polarization axis to the transparent axis of the polarized filter. Further, it is required to make the cost of the optical compensation sheet lower.
The optical compensation film often has optical defects. The production efficiency depends on incidence of the alignment defects in the optical compensation sheet so much.
Usually, a continuous sheet type of the film sheet base is used to produce a continuous type of the optical compensation sheet. When a film sheet for the optical compensation sheet has an alignment defect, the liquid crystal is aligned inadequately, which causes irregular unevenness (or mura) in a formed image: Accordingly the defective part of the optical compensation sheet can be used in neither polarizing filter nor the liquid crystal display, and therefore, the defective part of the optical compensation sheet is abolished.
The main causes of the alignment defects in the optical compensation sheet are bad conditions of the film sheet in the rubbing action. There are four main reasons of the bad conditions. First, a guide roller for feeding the film sheet damages the polymer layer, when a surface of the guide roller is not enough smooth, or when there are foreign particles between the film sheet and the guide roller. Secondly, an excess force is applied to the film sheet in the sheet roll. As the film sheet is rolled into the sheet roll, part of the film sheet contacts another part thereof with high stress and slides each other to rub and damage the surface of the polymer layer. Such damages more often occur in the middle part of the film sheet in the sheet roll.
Thirdly, when a surface of a reel for the sheet roll is not enough smooth, then the reel damages a surface of a forward edge of the film sheet. While the film sheet is rolled around the reel, the reel contacts the surface of the forward edge. Fourthly, when being set on a surface of the reel, the forward edge forms a step on the reel, and the step often causes to damage contact part of the polymer layer that is to in contact with the forward edge of the film sheet. These defects are formed intermittently frequently almost when in producing the continuous type of the optical compensation sheet.